I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a method and apparatus for providing an estimate of the signal strength of a received signal in a communication system.
II. Discussion of the Background
In many communication systems, the efficient use of system resource depends on the ability to accurately estimate the quality of the communication channel or link. The channel quality is related to the signal strength of a received signal which has been transmitted through the communication channel. Typically, the received signal comprises a desired signal and noise. The desired signal can be an information transmission signal or a signal representative of the information transmission, such as a pilot signal. The signal strength of the desired signal can be estimated and this signal strength can be used as an estimate of the signal strength of the received signal. Signal strength is defined as the signal-to-noise ratio SNR (the energy of the desired signal divided by the energy of the noise) or the signal-to-signal-plus-noise ratio S/N.sub.t (the energy of the desired signal divided by the total received energy).
A communication system can use the signal strength measurement to perform a variety of system optimization. First, the system can efficiently allocate resource based on the signal strength measurement. For example, the system can allocate more resource to transmissions on poor quality channels in order to maintain a requisite level or performance (e.g., a predetermined bit-error-rate). Alternatively, the system can allocate less resource to transmissions on poor quality channels in order to conserve resource. It can be determined that these transmissions are inefficient use of system resource since a small amount of information is transmitted versus a large amount of expended resource. Second, the signal strength measurement can be used to adjust the transmission rate to more efficiently utilize the allocated resource. For example, the transmission rate can be increased for transmissions on high quality channels and decreased for transmissions on poor quality channels. Third, the signal strength measurement can be used to more efficiently manage the communication system. As an example, consider a communication system which supports soft handoff. Soft handoff denotes the redundant transmissions from two or more source devices to a destination device to provide space diversity which can improve performance and reliability. The system can use the signal strength measurement to efficiently add or remove source device to or from the soft handoff transmissions. Soft handoff is described in U.S. Pat. No. 5,101,501, entitled "METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM", issued Mar. 31, 1992, and U.S. Pat. No.5,267,261, entitled "MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM", issued Nov. 30, 1993, both assigned to the assignee of the present invention and incorporated by reference herein.
In a typical communication system, the signal strength of a desired signal is computed from measurements of the energy of the desired signal and the energy of the noise. The energy of the desired signal can be determined by processing the received signal to remove the noise and computing the energy of the remaining desired signal. The energy of the noise can also be determined and computed. Alternatively, the energy of the noise can be set to a predetermined value by maintaining the amplitude of the received signal at a predetermined level through an automatic gain control (AGC) loop.
Accurate measurement of the signal strength is difficult because of a variety of factors. First, communication systems inherently operate in noisy environment. The noise increases the variation in the measurements of the energy of the desired signal and the energy of the noise. The variation in the measurements can be reduced by averaging the measurements over a longer time period. However, a tradeoff is made between the amount (or length) of averaging and the response time. Second, for mobile communication systems, the signal strength measurement is further complicated by variations in the channel characteristics due to the mobility of the communication devices. In a terrestrial environment, the transmitted signal can arrive at the destination device through one or more signal paths because of reflections and refractions from artifacts in the transmission path. The multiple copies of the received signal can add constructively or destructively at the destination device. A small displacement in the location of the destination device can result in a large change in the measured energy. This fading phenomenon can cause large variation in the signal strength measurement. And third, the signal strength measurements have a distribution density function which can be dependent on the structure of the signal and the statistic of the noise (e.g., an analog signal or a quadrature phase shift keying QPSK modulated signal) and the quality of the communication link.
For the foregoing reasons, a method and apparatus which can provide accurate estimates of the signal strength of a received signal in a communication system are much needed in the art.